Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a heating roller and a pressing roller pressed against the heating roller to form a nip therebetween through which the recording medium bearing the toner image is conveyed. As the recording medium passes through the nip, the heating roller heated by a heater and the pressing roller apply heat and pressure to the recording medium, thus melting the toner image on the recording medium. After the recording medium is discharged from the nip, the melted toner image is cooled and solidified on the recording medium. However, since the melted toner image is cooled by air at ambient temperature, the surface of the toner image is waved, applying insufficient gloss to the toner image.
To address this problem, a fixing device having a gloss application mechanism is proposed. For example, an endless belt formed into a loop is stretched over a plurality of rollers including a heating roller heated by a heater, which in turn heats the endless belt. A pressing roller is pressed against the heating roller via the endless belt to form a nip between the pressing roller and the endless belt. As a recording medium bearing a toner image is conveyed through the nip, the endless belt heated by the heating roller and the pressing roller apply heat and pressure to the recording medium, thus melting the toner image on the recording medium. A cooler, such as a heat sink, is disposed inside the loop formed by the endless belt and downstream from the nip in the conveyance direction of the recording medium. As the recording medium discharged from the nip and in contact with the outer circumferential surface of the endless belt is conveyed by the endless belt, the cooler contacting the inner circumferential surface of the endless belt cools the recording medium via the endless belt. Since the toner image on the recording medium is cooled and solidified while it contacts the smooth outer circumferential surface of the endless belt, the surface of the toner image is not waved, thus attaining desired gloss of the toner image.
On the other hand, it is requested that the endless belt is warmed up to a target temperature of about 160 degrees centigrade in a shortened time. However, it takes longer for the endless belt in contact with the cooler to be heated to the target temperature because the cooler draws heat from the endless belt. Moreover, it is necessary to liberate the heat drawn to the cooler so that the cooler is ready to cool the recording medium, resulting in decreased thermal efficiency.